This invention pertains to automated manufacturing environments, such as semiconductor manufacturing, and, more particularly, to a method and apparatus for determining scheduling priority using discrete event system simulation at the fabrication level.
Growing technological requirements and the worldwide acceptance of sophisticated electronic devices have created an unprecedented demand for large-scale, complex, integrated circuits. Competition in the semiconductor industry requires that products be designed, manufactured, and marketed in the most efficient manner possible. This requires improvements in fabrication technology to keep pace with the rapid improvements in the electronics industry. Meeting these demands spawns many technological advances in materials and processing equipment and significantly increases the number of integrated circuit designs. These improvements also require effective utilization of computing resources and other highly sophisticated equipment to aid, not only design and fabrication, but also the scheduling, control, and automation of the manufacturing process.
Turning first to fabrication, integrated circuits, or microchips, are manufactured from modern semiconductor devices containing numerous structures or features, typically the size of a few micrometers or less. The features are placed in localized areas of a semiconducting substrate, and are either conductive, non-conductive, or semi-conductive (i.e., rendered conductive in defined areas with dopants). The fabrication process generally involves processing a number of wafers through a series of fabrication tools. Each fabrication tool performs one or more of four basic operations discussed more fully below. The four basic operations are performed in accordance with an overall process to finally produce the finished semiconductor devices.
Integrated circuits are manufactured from wafers of a semiconducting substrate material. Layers of materials are added, removed, and/or treated during fabrication to create the integrated, electrical circuits that make up the device. The fabrication essentially comprises the following four basic operations:                layering, or adding thin layers of various materials to a wafer from which a semiconductor is produced;        patterning, or removing selected portions of added layers;        doping, or placing specific amounts of dopants in selected portions of the wafer through openings in the added layers; and        heat treating, or heating and cooling the materials to produce desired effects in the processed wafer.        
Although there are only four basic operations, they can be combined in hundreds of different ways, depending upon the particular fabrication process.
Efficient management of a facility for manufacturing products, such as semiconductor chips, requires monitoring of various aspects of the manufacturing process. For example, it is typically desirable to track the amount of raw materials on hand, the status of work-in-process and the status and availability of tools and tools at every step in the process. One of the most important decisions in controlling the manufacturing process is selecting which lot should run on each process tool at any given time. Additionally, most tools used in the manufacturing process require scheduling of routine preventative maintenance (PM) procedures and equipment qualification (Qual) procedures, as well as other diagnostic and reconditioning procedures that must be performed on a regular basis, such that the performance of the procedures does not impede the manufacturing process itself.
One approach to this issue implements an automated “Manufacturing Execution System” (MES). An automated MES enables a user to view and manipulate, to a limited extent, the status of tools, or “entities,” in a manufacturing environment. In addition, an MES enables the dispatching and tracking of lots or work-in-process through the manufacturing process to enable resources to be managed in the most efficient manner. Specifically, in response to MES prompts, a user inputs requested information regarding work-in-process and entity status. For example, when a user performs a PM on a particular entity, the operator logs the performance of the PM (an “event”) into an MES screen to update the information stored in the database with respect to the status of that entity. Alternatively, if an entity is to be taken down for repair or maintenance, the operator logs this information into the MES database, which then prevents use of the entity until it is subsequently logged back up to a production ready state.
Although MES systems are sufficient for tracking lots and tools, such systems suffer several deficiencies. Current MES systems largely depend on manufacturing personnel for monitoring factory state and initiating activities at the correct time. One technique for actively affecting the flow of lots through the fabrication process is to assign each lot a priority, which represents the importance assigned to completing the particular lot with respect to all other lots being fabricated. Generally, if multiple lots having different priorities seek to be processed by a particular entity, the lot with the higher priority is scheduled to be processed first. Typically, a particular lot may be expedited by manually increasing its priority, for example, to the highest priority.
Changes to lot priority can affect the production flow, especially the on-time delivery of other lots. Typically, after the priority of a lot is changed, it remains at the new level for the remainder of its fabrication. Hence, changing a particular lot to have the highest priority may result in the lot being completed much earlier than is actually required.
Changing lot priority is identified as a major disruption in the semiconductor Industry. Lots of the highest priority have the following characteristics: they are allowed to reserve downstream tools; they may be manually carried for both inter- and intra-bay transportation; they may have to be batched alone for lots of new technology, or they can be batched alone without waiting for other incoming lots; they may be allowed to break setup (i.e., change the recipe of a particular entity); and they may be allowed to break cascading (i.e., change recipe in a multi-chamber tool causing chamber idle time).
Although the number of lots of highest priority is typically very limited in the fabrication facility, they bring significant impacts to the production flow. For bottleneck tools (e.g. a photolithography stepper), the capacity loss of tool reservations by priority lots is not recoverable. Hence, the reserving of bottleneck tools directly reduces throughput. For batching operations, tool reservations or single lot batches appreciably increases the average cycle time of production lots, as they may have to wait an extended time period (e.g., 12 hours) to get the tool. Such reservations also reduce tool utilization (e.g., a furnace is typically capable of processing six lots simultaneously, but only one priority lot in the same amount of time if a reservation is made). For steppers, priority lots also reserve reticles when reserving the tool, which expands the effect to other tools that may have to unload the reticle sets and experience unnecessary setups. As priority lots break cascading, it reduces the throughput of the overall manufacturing facility as well as increases the fluctuation of production flow.
Hence, changing lot priority is challenging because the effects of changing lot priorities are felt across the whole production environment and have an extended time horizon. In other words, the effects are not instantaneous and can last as long as a few months. Also, the effects of lot priority changes present different issues for different types of tools. Determining lot priorities requires both a global view of the fabrication facility and local views at the tool level to consider their correlation. Further, lot priority should be determined with the consideration of other priority lots and their status.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.